Laser processing technology has been gradually developing toward ultra-precision, ultra high-speed, and large scale for processing of precision components in the electronics industry. In particular, ultra-precision processing is required for processing parts in the field of microelectronics including semiconductors, displays, solar cells, next-generation high value/high performance PCBs, and next-generation packages.
For such ultra-precision processing at micro-scale, high laser performance is also required. In order to achieve ultra-precision processing, a method using a laser beam in an ultraviolet region or an ultra-short pulse laser beam of a short pulse width is employed. In addition, a pulse laser having high repetition rate and high output is also required for high speed and large scale laser processing.
However, since current pulse lasers employ a very complex optical system, cannot achieve high repetition rate and high output, and have lower operation stability than existing ultraviolet nanosecond pulse lasers, the pulse laser does not have industrial applicability. Thus, an ultraviolet (UV) laser having high output and high repetition rate is needed as a light source for ultra-precision/high speed processing.
Generally, a UV laser system produces an infrared laser beam of high output and oscillates the infrared laser beam into a UV laser beam through nonlinear wavelength conversion. To this end, the laser system converts output pulses having the same interval generated in an oscillator into desired time-based output pulses (burst mode) through time-based selection using various forms of optical modulators. Hereinafter, an internal structure of a bust mode oscillating apparatus of a typical laser system will be described with reference to FIG. 1.
FIG. 1 is a block diagram of an internal structure of a burst mode oscillation apparatus of a typical laser system.
Referring to FIG. 1, the burst mode oscillation apparatus include an oscillator 101, a main amplifier 102, an optical modulator 103, and a nonlinear wavelength converter 104. The oscillator 101 receives an electrical signal including pulse widths of the same size, oscillates output light in a pulse unit from the received electrical signal, and transmits the oscillated output light to the main amplifier 102. The main amplifier 102 amplifies the pulse light oscillated by the oscillator 101, and transmits the amplified pulse light to the optical modulator 103.
The optical modulator 103 optically modulates the pulse light amplified by the main amplifier 102, and outputs the optically modulated light. Here, since the optical modulator 103 selects pulse signals generated by the oscillator 101 and the main amplifier 102 based on time, and outputs only the pulse signals corresponding to selected times, only a time-based pulse output control can be allowed. Further, since the optical modulator 103 can control the characteristics of output pulses having only an oscillating frequency or lower, a basic pulse width to be output cannot be varied.
In addition, when the optical modulator 103 varies pulse signals amplified by the oscillator 101 and the main amplifier 102 at a relatively low repetition rate, since temporarily high light signals are generated per unit time due to a low repetition rate by a rear optical amplifier, various optical parts constituting an optical amplifier are physically damaged, thereby making it difficult to freely vary the pulse repetition rate. Moreover, the optical modulator 103 must be additionally installed outside a laser oscillator, thereby significantly increasing installation costs. The nonlinear wavelength converter 104 receives a pulse signal optically modulated by the optical modulator 103, converts the wavelength of the received optically modulated pulse, and outputs light having the converted wavelength.
To solve such problems in the art, a master oscillator power amplification (MOPA) technology is introduced. MOPA employs a main oscillator and an amplifier in order to directly modulate time characteristics of light using a semiconductor laser and amplify the modulated light into high output signals. MOPA provides an advantage of facilitating modulation of pulse widths and frequency modulation, but has problems of output stability of a laser system and physical damage of the amplifier upon operation in a bust mode.